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If I didn't update portage often enough and then dared to run the update all hell used to break loose.
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I run stable AMD64, and it's been very stable. But, as you imply, leaving it too long between updates can lead to grief. I try to update once a fortnight, and suspect that monthly wouldn't be a problem.....After updating (a) run etc-update or any of its more user-freindly pals (I use etc-proposals).....Always read and if necessary act on news items.....Check your emerge logs (I use elogviewer, and deselect the setup messages).....keep your kernel up to date.....If you use portage-2 you get a couple of useful sets to emerge after updating the kernel [or xorg server] -
@module-rebuild
@x11-module-rebuild

I run pure ~amd64 on a Sandy Bridge i5 using Intel HD3000 gpu graphics with 16GB memory and couple of SSDs and a couple of big TB drives. I run gentoo on one partition and funtoo-current-corei7 on another, sharing a common home directory, and both distros are very reliable. I also run funtoo-current-atom on a little Intel Atom system with only 3GB memory. On gentoo I unmask portage-2.2.0_alpha* and on funtoo portage-2.3.5 is current. Both are running with the pf 3.6.9 kernel [bfs/bfq]. Gentoo unstable and Funtoo current are so rock solid in these installations that it sometimes seems to be impossible to break my system Of course I stay far away from anything tainted by gnome or systemd dependencies. I update daily and keep the two distros, gentoo and funtoo, in sync with same versions of KDE, openbox, xmonad, rox filer, firefox, emacs, gpodder, handbrake, claws-mail, R, octave, etc.

I used Gentoo years ago and then spent several years running NetBSD and then Archlinux. I tried to switch back to gentoo once or twice and was quickly frustrated at not even being able to complete an install without encountering blockers that I didn't know how to resolve. I think that gentoo has improved greatly, and I've learned better how to use the portage tools. The package maintenance tools in gentoo are almost perfectly engineered. I'm especially amazed at the good feedback that portage gives making it easy to do the right thing. e.g. after updating xorg server it suggested to me that I should either run
emerge @x11-module-rebuild
or
emerge -1 $(qlist -IC x11-drivers/)

When an update leaves libraries in an half-upgraded state portage simply reminds me at the end of the update that I need to run
emerge @preserved-rebuild

I've recently done full installs and week-long evaluations of OpenSUSE, Calculate, Chakra, Sabayon, Ubuntu, Kubuntu, as well as extensive experience with Archlinux. In my experience it would be harder to break a gentoo system than it is to break any of the binary distros, and if you or upstream did break something the gentoo maintenance tools would make it possible if not easy to fix the problem, e.g. by downgrading gracefully. Downgrading is not something you want to even try with most distros.

It seems from reading the various distro forums that upstream updates are creating more problems than they have in years. KDE is in a very stable period but gnome and some system libs are churning. At least gentoo has a good bug tracking system and a clear policy on when a package can be released to ~amd64 and when it can be accepted as stable.

what's the fun of linux if there aren't problems to solve, breaks to fix?

i've come back to gentoo after a while off from linux and i find that things are stable here with gentoo. took me a while to tweak my kernel to the current state of perfection, but it's pretty smooth sailing once you get a kickass kernel config.

portage is very good if you want/need/enjoy compiling your own system. only arch linux's pacman is in this league.